As a representative of the third generation of semiconductor materials, gallium nitride (GaN) has many excellent characteristics, such as high breakdown field, high electron mobility, high two-dimensional electron gas concentration, and good thermal stability, etc. The semiconductor devices based on gallium nitride, such as high electron mobility transistor (HEMT), heterostructure field effect transistor (HFET) and the like, have been put into use, and have obvious advantages especially in areas where high power and high frequency are required, such as radio frequency and microwave, etc.
GaN materials have two polarities along a C-axis, i.e. an N-face polarity and a Ga-face polarity. GaN in most HEMT devices has the Ga-face polarity, while N-face polarity GaN is based on the opposite crystalline orientation. The advantages of N-face polarity GaN include: a) better two-dimensional electron gas (2DEG) carrier confinement mechanism under negative bias conditions, which can form better off-state pinch off through back barrier; b) since the metal electrode is directly disposed on the conductive channel layer (such as GaN or InGaN material), an ohmic contact with a lower resistivity can be obtained, and the ohmic contact on GaN material has a lower Schottky barrier than the ohmic contact formed on AlGaN material in conventional HEMT devices. However, how to grow GaN with N-face polarity has always been a difficulty, and there is still no suitable process to produce HEMT devices with N-face polarity GaN.